Office chairs are known comprising a spoke-type base with feet or wheels, a column mounted on said base and provided with a gas piston for adjusting the height of the sitting plane from the floor, and a seating portion which can be separate from a seat back portion or joined to it and supported by said column.
Generally the seat back portion consists of a pair of tubes which engage telescopically, one tube being rigid with the seating portion and the other tube being provided with a padded back-rest.
A known type of height adjustment device for the back-rest consists of a locking pin rigid with one of the tubes and slidable within a slotted hole provided in the other tube.
In another known device consists of a pin engagable in a hole provided in one tube and engagable elastically in a plurality of superposed holes provided in the other tube.
To another known type of adjustment device, one of the telescopic tubes comprises a plurality of notches in which a rigid tooth element elastically engages.
These known devices have however certain drawbacks, and in particular:
laborious construction due to the large number of components involved, PA0 laborious assembly, which is generally effected in the factory and results in a substantial space requirement during transport, PA0 laborious operation in adjusting to the desired height, PA0 unreliable adjustment as it is related to the distance between the holes. PA0 a guide element rigid with a support plate of a seating portion, PA0 a slide for supporting said back-rest, said slide being engagable with said guide element and slidable along it, PA0 means which are elastically selectively engageable with a toothed portion to ensure the stability of the position of said slide relative to said guide,